In non-contact measurement systems for measuring the physical characteristics of three dimensional (3D) objects such as turbine blades used in aircraft engines, one problem often encountered is accurately locating the edges of the object.
In acceptance testing of parts having a complex part shape such as, for example, a compressor blade, it is often not possible to obtain accurate information using conventional hard gauging techniques. Rather, a three dimensional (3D) surface measurement technique is employed using a structured light system. In such systems, the part under test is illuminated with a series of striped laser lines with surface feature information being mapped or registered into a coordinate system. The results are then compared with, for example, a CAD definition of the part to determine its acceptability.
In using this 3D measurement technique, one problem encountered is accurately locating the edges of the part. In this regard, it will be appreciated that the edges are not necessarily sharp edges, but rather are radiused; i.e., they have a rounded contour. Conventional 3D systems do not locate the edges of a radiused part well. If the location of an edge (or edges) cannot be precisely found, then the location of other surfaces features of the part in the co-ordinate system cannot be established with the necessary certainty to insure that accurate part data is obtained on which to accept or reject the part.
Heretofore, when 3D systems have been used for testing purposes, location of an edge has been attempting by doing a best fit of the data obtained from by the viewing system. While this approach is useful when edges are sharp, it is not effective when the edge is gently curved. An alternate approach has been to backlight the part and locate the edge through the resulting profile created when as part is turned on a pedestal or support. This method not only requires additional processing of the image data obtained by the viewing system, but the results obtained are not as accurate as those from the 3D method itself.
The method and apparatus of the present invention incorporate a diffuse (white) light source into a structured (laser) light system to obtain highly accurate edge location information so to provide more reliable part testing results.